2003 GMC Sonoma Owner's Manual - Page 86

Leaving Your Vehicle If you are leaving the vehicle, take your keys, open your door and set the locks from inside. Then get out and close the door. Your vehicle may be equipped with a third door for convenience in loading and unloading cargo. To open the third door, first open the driver's front door. Then, use the handle located on the front edge of the door panel to open it. You must close the third door before you can close the driver's door. Third Door 2-11